Implemented one-char cache lookup in generated code.

test/mjsunit/string-charat.js

Index: test/mjsunit/string-charat.js
diff --git a/test/mjsunit/string-charat.js b/test/mjsunit/string-charat.js
index 8ec8f1e08f5b3b7c40ecffaabac410f45b9db378..d1989dfd731177e50c4bd3dcbdd585995901b16e 100644
--- a/test/mjsunit/string-charat.js
+++ b/test/mjsunit/string-charat.js
@@ -51,3 +51,16 @@ assertEquals(116, s.charCodeAt(NaN));
 assertTrue(isNaN(s.charCodeAt(-1)));
 assertTrue(isNaN(s.charCodeAt(4)));
 
+// Make sure enough of the one-char string cache is filled.
+var alpha = ['@'];
+for (var i = 1; i < 128; i++) {
+  var c = String.fromCharCode(i);
+  alpha[i] = c.charAt(0);
+}
+var alphaStr = alpha.join("");
+
+// Now test chars.
+for (var i = 1; i < 128; i++) {
+  assertEquals(alpha[i], alphaStr.charAt(i));
+  assertEquals(String.fromCharCode(i), alphaStr.charAt(i));
+}